UV LEDs are really dim. I've got some 1.8mm ones. :/
I can't speak about yellowing of keycaps though. I know that Hoff used 3mm UV LEDs for his previous GON build.
Ah, very good to know! If they are truly dim then they won't make good backlights. I suppose smaller resistors could be used to attempt to crank up the current, within reason.
I'm thinking about 405nm LEDs for a new build. They were very expensive but I decided to just pull the trigger anyway. If I don't use them then they will find a home in my electronics kit.
As for the yellowing of caps, that doesn't worry me at all. An LED couldn't possibly match the UV emission of the Sun, and my keyboard has a decent view out the window every single day.
I've got to find a photo of this board you speak of.